1) Field of the Invention
This invention relates to a metalization structure that uses a polyimide which has low dielectric constant, low thermal expansion coefficient, high glass transition point and high heat resistance as an insulating material, in particular, a semiconductor device or a multilevel metalization structure that have a high level of integration.
In another aspect, the invention relates to a polyimide that has a low thermal expansion coefficient, a high glass transition point and high heat resistance and which is capable of strong bonding by itself through oxygen ashing. The invention also relates to a precursor of such polyimide, as well as a metalization structure that uses said polyimide as an insulating material, in particular, a semiconductor device or a multilevel metalization structure that have a high level of integration.
2) Description of the Related Art
As increasing efforts are being made to increase the number of layers and the scale of integration in recent models of electronic devices such as semiconductor devices, thereby improving their performance, it has become necessary for the insulating materials used in those devices to possess enhanced characteristics. Most of the insulating materials extensively currently being used in electronic devices are polyimides. Before the advent of polyimides, inorganic films such as SOG, PSG and silicon nitride films were used but they had the problem that it is difficult to flatten out the asperities that occur in the process of fabricating semiconductor devices. Another problem is that these inorganic films are insufficient in mechanical characteristics, particularly, elongation, and henc cracks are prone to occur in those areas where residual stress has developed between layers. To solve these problems, polyimides were introduced and have since been extensively used in the electronics industry.
Polyimides are generally manufactured by a process in which a diamine component is reacted with a tetracarboxylic dianhydride component in an organic solvent to produce a poly(amic acid), which is then cyclized by dehydration.
Polyimides known in the art that are synthesized by this general method include the following.
(A) Novel polyimides containing structural units represented by the general formula (10) or (11): ##STR4## (where R' is a divalent hydrocarbon group), as well as their precursor poly(amic acid) or poly(amic acid) ester (see Unexamined Published Japanese Patent Application Nos. 265327/1987 and 10629/1988);
(B) Polyimides containing structural units represented by the general formula (12): ##STR5## (where R" is a tetravalent aliphatic group or an aromatic group; and n is 1 or 2) (see Unexamined Published Japanese Patent Application Nos. 114258/1982, 188853/1982, 250031/1985 and 221426/1985);
(C) Polyimides containing structural units represented by the general formula (13): ##STR6## (where Y is --C(CH.sub.3).sub.2 --, --C(CF.sub.3).sub.2 -- or --SO.sub.2 --) (see Unexamined Published Japanese Patent Application Nos. 231935/1987, 31936/1987 and 231937/1987);
(D) Polyimides having low dielectric constant that are produced by reacting 2,2-bis(3,4-dicarboxyphenyl)propanoic dianhydride or 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropanoic dianhydride with aromatic diamines such as 4,4'-bis(4-aminophenoxy)biphenyl and 4,4'-bis(4-amino-2-trifluoromethylphenoxy)biphenyl (see Unexamined Published Japanese Patent Application No. 60934/1990); and
(E) Polyimides produced by reacting 2,2-bis(4-aminophenyl)hexafluoropropane and 2,2-bis(3-aminophenyl)hexafluoropropane with a mixed acid dianhydride composed of pyromellitic dianhydride and an acid dianhydride having a diarylic nucleus (see Unexamined Published Japanese Patent Application No. 67320/1990), as well as polyimides produced by reacting 2,2'-bis (3,4-dicarboxyphenyl)hexafluoropropanoic dianhydride with 2,2-bis(4-aminophenyl)hexafluoropropane and 2,2-bis(3-aminophenyl)hexafluoropropane (see Unexamined Published Japanese Patent Application No. 86624/1990).
The polyimides (A)-(E), however, have the problem that various desired characteristics including high heat resistance, low dielectric constant, low thermal expansion, good mechanical properties (in particular, high flexibility) and high glass transition point are not given equal consideration. Furthermore, the polyimides (D) and (E) which contain trifluoromethyl groups bound to alkyl chains suffer from the disadvantage of only small ability to withstand organic solvents and alkaline solutions such as electroless plating baths. Hence, the use of those polyimides in fabricating electronic devices such as semiconductor devices and multilevel metalization structures can potentially involve certain problems such as limits to the effort to improve the device performance and difficulties that may be encountered in the implementation of fabrication process.
The polyimides (A)-(C) are satisfactory in terms of heat resistance, thermal expansion and glass transition point but, on the other hand, they have a high dielectric constant in the absence of flexibility. The reason for this may be that: those polyimides have a relatively high content of imide rings in the polymer and contain structural units represented by the general formula (14): ##STR7##
The polyimides (D) and (E) which contain both --CF.sub.3 (triftuoromethyl) groups and --O-- bonds are low in dielectric constant and high in flexibility but, on the other hand, they are low in heat resistance and glass transition point, high in thermal expansion coefficient and have only small capability of withstanding alkaline solutions and organic solvents.
If insulating films have high dielectric constant, the delay time of signals propagating through the metalization will increase to lower the speed of signal propagation. Further, the films will become highly hygroscopic, increasing the chance of debonding which can potentially cause corrosion in the metalized part, leading to an increased current leakage. To avoid these problems, the dielectric constant of insulating films is desirably as low as possible. If insulating films have high thermal expansion coefficient, thermal stress will develop between the metalized part and the substrate, potentially causing various problems such as bow of the substrate, separation of the insulating film, occurrence of cracks and broken metalizations. To avoid these problems, the thermal expansion coefficient of insulating films is desirably close to those of the substrate and the metalized part. If insulating films have low glass transition point, their thermal expansion coefficient is high and the temperature at which the expansion coefficient starts to deviate from the values of the metalized part and the substrate will decrease, thereby developing a greater thermal stress. To avoid this problem, the glass transition point of insulating films is desirably as high as possible. If insulating films do not have satisfactory heat resistance, the process operating temperature cannot be adequately increased. If insulating films do not have adequate flexibility or elongation, the thermal stress that may occur cannot be sufficiently absorbed to prevent the separation of insulating films, the occurrence of cracking and broken metalizations.
As the number of layers used in electronic devices increases, the thermal expansion coefficient of polyimide and other organic insulators used in the devices must accordingly be reduced. This is because the thermal expansion of organic insulating films is generally from several to several tens of times as great as that of metallic materials that form metalization or inorganic materials that provide the substrate and this thermal expansion mismatch will cause several disadvantages. First, great thermal expansion mismatch between the metalization material and the insulating material will cause stresses to develop between the two materials, causing broken metalizations or cracked insulating films, thus leading to failures or lower device reliability. Second, the great thermal expansion mismatch between the substrate material and the insulating material will cause so great a bow of the substrate under stress that patterning such as photoetching of the upper layers cannot be accomplished very precisely and the resulting difficulties in fabrication process will lead to failures and lower device reliability.
With a view to dealing with these problems, polyimides of low thermal expansion have been proposed in Unexamined Published Japanese Patent Application Nos. 114258/1982, 188853/1982, 250031/1985 and 221426/1985. Other examples of such polyimides have been described in Unexamined Published Japanese Patent Application Nos. 60725/1986, 184025/1987 and 232436/1987. However, none of those polyimides have been given consideration for the property of "adequate adhesion" which is regarded as one of the necessary and indispensable characteristics for fabrication of electronic devices. Generally speaking, polyimides having low values of thermal expansion coefficient .alpha. (.ltoreq.20 ppm/.degree.C.) adhere only weakly to substrates, metallic materials and the polyimides themselves compared to polyimides of high thermal expansion (.alpha..ltoreq.40 ppm/.degree.C.) and, hence, debonding is likely to occur at the interface between the polyimide and the substrate, metalization material or the polyimide itself.
The polyimides of low thermal expansion just mentioned above are not given any consideration for the property of "adhesion" to the substrate, metallic material or the polyimides themselves. If the adhesion is not satisfactory, debonding is prone to occur at all kinds of interfaces and the resulting entrance of water can be a cause of corrosion of the metalized part, leading to lower device reliability or various difficulties involved in the fabrication process to render the completion of electronic devices impossible.